Spring powered gas operated weapon

ABSTRACT

A piston head ( 1 ) suitable for a spring powered gas operated weapon such as an air rifle or air pistol is disclosed. The piston head ( 1 ) is comprised of a piston end and a spring abutment means in which the piston end is comprised of a head ( 5 ) and a plunger ( 3 ), the spring abutment means is comprised of a body ( 2 ) and an extended boss ( 4 ). At least the body ( 2 ) of the spring abutment means defines a blind bore ( 20 ) which extends from the surface of the body ( 20 ) remote from the extended boss ( 4 ) and towards the extended boss ( 4 ) and preferably into the extended boss ( 4 ). The plunger ( 3 ) and blind bore ( 20 ) are dimensioned to allow the plunger ( 3 ) to fit within the blind bore ( 20 ), and the plunger ( 3 ) is provided with at least one means for mounting a seal means ( 6 ) around the plunger ( 3 ) to allow the creation of a seal between the plunger ( 3 ) and the or each wall defining the blind bore ( 20 ).

This invention relates to a spring powered gas operated weapon, in particular to a piston head for use in a spring powered gas operated weapon and spring powered gas operated weapons containing such a piston head.

Spring powered gas operated weapons such as air rifles and s air pistols are well known. In essence such weapons are comprised of a barrel, a compression cylinder and a trigger mechanism. Within the compression cylinder is located a helical compression spring with a piston head attached to a first end of the spring. A means is provided for compressing the spring at a first end of the compression cylinder and the trigger mechanism holds the spring in its compressed form. The spring is orientated so that the piston head is at the end of the spring remote from the first end of the compression cylinder when the spring is compressed.

A first end of the barrel mouths on to the second end of the compression cylinder either directly or indirectly at or close to the second end of the compression cylinder. The projectile to be fired by the weapon is placed within the barrel, normally at or adjacent the first end of the barrel.

When the weapon is fired, the trigger mechanism is activated and the spring is released. The spring starts to decompress and drives the piston head along the compression cylinder toward the second end of the compression cylinder. The piston head pushes the air within the compression cylinder toward the second end of that cylinder. The increased air pressure in the compression cylinder pushes the projectile along the barrel away from the first end of the barrel until the projectile exits the barrel at the second end of the barrel. To transfer as much energy from the compressed air to the projectile, it is known to provide projectiles that fit closely but slidingly within the barrel. If the projectile is too loosely fitting in the barrel the air in the compression cylinder may pass around the projectile in the barrel and the energy released when the compression spring decompresses is not transferred to the projectile.

A problem encountered with spring powered gas operated weapons is that as the compression spring decompresses and pushes the piston head along the compression cylinder the air pressure in front of the piston head increases until the air pressure is sufficiently high to slow and then stop the decompression of the spring and thus the movement of the piston head. Assuming that the projectile is correctly sized for the barrel, at the same time as or a very short time after the time the air pressure stops the spring decompressing the projectile starts moving along the barrel so increasing the combined volume of the cylinder and part of the barrel between the first end of the barrel and the projectile. This causes the air pressure in that volume to reduce and will allow the spring to start decompressing again so starting the piston head to start moving toward the second end of the cylinder. This effect is known as piston bounce.

Once the piston head reaches the second end of the compression cylinder it impacts the second end of that cylinder. This impact both stops the forward movement of the piston head and, because helical compression springs tend to rotate about their central axis when decompressing, transmits some rotational movement or torque to the end of the compression cylinder.

The user of the spring powered gas operated weapon thus feels a number of shocks in the weapon as it is being fired. Because the nature of spring powered gas operated weapons the projectile to be fired is located in the barrel for a short time after the trigger is activated and the projectile is generally still in the barrel when the shocks from the piston bounce and the piston hitting the end of the cylinder are experienced. This can lead to movement of the spring powered gas operated weapon making it less likely that the intended target will be hit.

According to the present invention there is provided a piston head suitable for a spring powered gas operated weapon characterised in that

the piston head is comprised of a piston end and a spring abutment means the piston end is comprised of a head and a plunger, the spring abutment means is comprised of a body and an extended boss, at least the body of the spring abutment means defines a blind bore which extends through at least the body where the blind bore extends from the surface of the body remote from the extended boss and towards the extended boss, the plunger and blind bore are dimensioned to allow the plunger to fit within the blind bore, and the plunger is provided with means for mounting a seal means around the plunger to allow the creation of a seal between the plunger and the walls defining the blind bore.

It is most preferred that the body and extended boss of the spring abutment means both have substantially cylindrical curved surfaces and the cylindrical curved surfaces of the body and extended boss have a substantially common central axis. The body and the extended boss are preferably both cylindrical but they may, in other embodiments of the present invention, have end faces that are not flat. The end faces may, for example be rounded or conical.

It is most preferred that the blind bore in the spring abutment means is defined by a longitudinally extending surface that is substantially cylindrical. Most preferably the axis of that cylinder is substantially common with the axis of the body and the extended boss, that is each of the body, extended boss and blind bore have substantially the same central axis. This configuration is particularly preferred because the compression cylinders of spring powered gas operated weapons are generally cylindrical in nature and the symmetry of the spring abutment means of the present invention about its central axis has the effect that the spring abutment means of the present invention does not need to specially orientated when it is located within the compression cylinder of an spring powered gas operated weapon.

As with the spring abutment means of the present invention, it is preferred that the head and the plunger of the piston end both have substantially cylindrical curved surfaces and the cylindrical curved surfaces of the head and the plunger have a substantially common axis. Again this is because the compression cylinders of spring powered gas operated weapons are generally cylindrical in nature and the symmetry of the spring abutment means of the present invention about its central axis has the effect that the spring abutment means of the present invention does not need to be specially orientated when it is located within the compression cylinder of a spring powered gas operated weapon.

It is most preferred that when the plunger is within the blind bore the body, extended boss, piston end and plunger all have a substantially common axis.

It is most preferred that the radii of the substantially cylindrical surfaces of the boss of the spring abutment means and the head of the piston end are substantially the same. In embodiments of the present invention where this is so, there is a particular benefit because it has the effect that when the piston head of the present invention is within the compression cylinder and being pushed along the compression cylinder by the decompressing spring, the substantially cylindrical faces of the boss of the spring abutment means and the head of the piston end will both be in sliding engagement with the wall of the compression cylinder so creating a double seal between the piston head and the wall of the compression cylinder. This leads to less leakage of air around the piston head than in currently known piston heads and thus a greater transfer of energy to the projectile being fired.

Such an increase in the transfer of energy to the projectile can have the result that for the same spring as would be used in connection with a known piston head in a known spring powered gas operated weapon the projectile will travel faster. Alternatively, this effect can be used to allow a manufacturer of a spring powered gas operated weapon to use a smaller and or lighter (less powerful) spring to achieve the same performance as is achieved by currently known spring powered gas operated weapons. This has the advantage that less material is used in the spring leading to cost and weight savings.

It is a particular advantage of the piston head of the present invention that because in the most preferred embodiments of the present invention both the piston end and the spring abutment means are cylindrical they can be fitted to existing spring powered gas operated weapons without the need to alter the compression cylinders of those weapons.

The extended boss of the spring abutment means is most preferably dimensioned so that it may extend into the space that extends along the length of a helical compression spring between the coils of the spring. Most preferably the dimensions of the helical spring with which the piston head will be used are known and the cylindrical face of the extended boss will be dimensioned to just fit within the space along the core of the compression spring. This is beneficial because it means that the extended boss will help hold the piston head in a fixed position relative to the end of the compression spring, and the overall length of the combined spring and piston head will not be much greater than that of known spring and piston head combinations.

A further advantage of having the extended boss project into the space within the compression spring is that it means that the end of the compression spring will bear on the body of the spring abutment means. Thus only the body of the spring abutment means needs to be engineered to withstand the forces which will be exerted by the compression spring. The extended boss can thus be lighter and use less material than would be required if the spring acted on to the extended boss.

In preferred embodiments of the present invention the blind bore in the spring abutment means is defined by the body and extended boss, that is the blind bore extends through the body and into the extended boss. This is preferred because it allows the body of the spring abutment means to be long enough, in the direction of the central axis of the blind bore, to have sufficient strength to resist the forces it will experience from the compression spring, but not so long as to define all of the blind bore. Typically this length of the body may be as little as 2.0 mm.

It is most preferred that the depth of the blind bore defined by the spring abutment means is equal to or greater than the length of the plunger of the piston end, most preferably it is greater than the length of the plunger of the piston end.

The plunger and the blind bore are preferably both so dimensioned and configured that the plunger closely fits within the blind bore. In this context, closely fitting means that the plunger can move into and out of the blind bore with ease but the dimensions in a direction perpendicular to the central or longitudinal axis of the plunger and blind bore are such that very little lateral movement (in a direction perpendicular to the central or longitudinal axis of the plunger and blind bore) is possible. Preferably that lateral movement is equal to or less than 2 mm, equal to or less than 1.0 mm, equal to or less than 0.5 mm, or equal to or less than 0.2 mm.

The plunger is provided at least one means for mounting a seal means around the plunger to allow the creation of a seal between the plunger and the or each wall defining the blind bore. It is most preferred that the or each means for mounting a seal means around the plunger is at least one groove or gland (hereafter referred to as grooves). Most preferably there are one, two, three or four grooves.

It is particularly preferred that at least one groove is closer to the end of the plunger remote from the piston end than it is to the piston end.

In such preferred embodiments, a suitably sized O-ring is located in at least one of the grooves. O-rings are a well known technology and an appropriate material for the O-ring may be readily chosen.

It is most preferred that for at least one of the grooves, the groove has a cross-sectional profile such that the grove has first and second side walls and a base, in which both side walls intersect with the base and extend there from to the surface of the plunger, the first side wall is closer to the piston end than the second side wall, and the first side wall is planar and substantially perpendicular to the longitudinal axis of the plunger.

It is preferred that the second side wall is planar and not parallel to the first side wall and the second side wall is planar and closest to the first side wall at the second side wall's intersection with the base of the groove. It is most preferred that the angle between the second side wall and the perpendicular to the longitudinal axis of the plunger is in the range of about 30° to 70°, more preferably of about 40° to 60° and most preferably of about 50° to 60°.

The base of the groove may, in section, be flat, but most preferably it is concave.

The mouth of the blind bore is in a surface of the body of the spring abutment means, most preferably in the surface remote from the enlarged boss of the spring abutment means. The intersection of the blind bore and the surface of the body of the spring abutment means is preferably chamfered, bevelled, or rounded.

In preferred embodiments of the present invention the piston end further comprises an energy absorbent material surrounding the plunger adjacent the intersection of the plunger and the head of the piston end. This energy absorbent material is a material that readily absorbs kinetic energy such as an elastomer. The purpose of the energy absorbent material is to stop the body of the spring abutment means impacting on the head of the piston end when, for example, the piston end impacts on the end of the compression cylinder (an event that is likely to send shock waves through the spring powered gas operated weapon into which the present invention is incorporated). It is most preferred that the energy absorbent material surrounding the plunger is an O-ring of a known material.

It will be readily appreciated that the piston head of the present invention can be easily incorporated into a spring powered gas operated weapon of known design. The incorporation is easily achieved by substituting the piston head of the present invention for the piston head in the known design. All that is required for a successful substitution is to ensure that the dimensions of the piston head of the present invention are appropriate for the compression cylinder of the spring powered gas operated weapon. The substitution can be made either during construction of the spring powered gas operated weapon or at a later date. Such a substitution will create a spring powered gas operated weapon comprising a barrel, a cylinder, a helical compression spring, and a trigger mechanism, in which the compression spring is located within the cylinder, and a piston head according to the present invention is attached to a first end of the spring.

It is most preferred that such a substitution results in an spring powered gas operated weapon comprising a barrel, a cylinder, a helical compression spring, and a trigger mechanism, in which the compression spring is located within the cylinder, characterised in that a piston head according to the present invention which comprises a cylindrical boss and a piston end is attached to a first end of the spring, the radii of the substantially cylindrical surfaces of the boss and piston end are substantially the same as each other, and the substantially cylindrical surfaces of the boss and piston end are in sliding contact with the cylindrical surface defining the compression cylinder of the spring powered gas operated weapon.

Use of the piston head of the present invention in a spring powered gas operated weapon is advantageous because it has been found that the plunger and blind bore of the piston head together act as a damper. The damper absorbs the piston bounce effect experienced with spring powered gas operated weapons. Furthermore, because in the most preferred embodiments of the present invention the plunger may rotate about it's central axis whilst within the blind bore, when the piston head encounters the end of the compression cylinder the rotation or torque exerted by the compression spring is applied to the spring abutment means, which rotates independently of the piston end.

Further advantages of the piston head of the present invention are that because of the increase in the energy transmitted to the projectile for the spring, a lighter spring can be used to achieve the same results as currently known designs. The use of a lighter spring means that less force is required to compress that spring prior to firing. In the majority of known designs of spring operated spring powered gas operated weapons, compression of the spring is performed by the user using the barrel as a lever to gain sufficient mechanical advantage to be able to compress the spring, The need for less force to compress a spring in an spring powered gas operated weapon that incorporates a piston head of the present invention means that less mechanical advantage is need to compress the spring so the lever or barrel may be shorter than in known designs. A shorter barrel is advantageous because it uses less material, it weighs less, and the time the projectile is in the barrel after firing, the lock time, is less. The decrease in lock time means that the user has to maintain their aim at the target for less time and thus increases the accuracy of the user's shooting.

The piston head of the present invention will be further described and explained by way of example with reference to the accompanying drawings in which:

FIG. 1 shows a first embodiment of a piston head according to the present invention;

FIGS. 2 to 4 show front, side and back views of the piston head of FIG. 1;

FIG. 5 shows the piston head of FIG. 1 in partial section, fitted in the compression cylinder of a spring powered gas operated weapon;

FIG. 6 shows an exploded view of a second embodiment of a piston head according to the present invention;

FIG. 7 shows the piston head of FIG. 6 in section; and

FIG. 8 shows a detail of the piston head of FIG. 6.

FIG. 1 shows the piston head (1) comprising a cylindrical body (2) with a cylindrical plunger (3) inserted at one end and a smaller diameter cylindrical boss (4) at the other end, the cylindrical plunger (3) has a head (5) equal to the diameter of the cylindrical body (2) at the opposite end to the cylindrical boss (4).

FIG. 2 shows a front view of the piston head (1) wherein the cylindrical head (5) of the plunger (3) is visible and like parts have like reference numerals.

FIG. 3 shows a side view of the piston head (1) comprising of a cylindrical body (2) with a cylindrical plunger (3) inserted at one end and a smaller diameter cylindrical boss (4) at the other end, the plunger (3) has a cylindrical head (5) equal to the diameter of the cylindrical body (2) at the opposite end and like parts have like reference numerals.

FIG. 4 shows the back view of the piston head (1) wherein the cylindrical body (2) and smaller diameter cylindrical boss (4) are visible and like parts have like reference numerals.

FIG. 5 shows the piston head (1) in section, wherein two O-ring seals (6) are visible on the cylindrical plunger (3) which is inserted into a fluid reservoir and/or a compressible polymeric material (7) within a hollow section of the cylindrical body (2), piston head (1) is fitted within a spring powered spring powered gas operated weapon compression cylinder (8) wherein the smaller diameter extended boss (4) of the cylindrical body (2) receives a spring (9) and the head (5) equal to the diameter of the cylindrical body (2) but up against the spring powered gas operated weapon piston seal (10).

In a second preferred embodiment of the present invention illustrated in FIGS. 6, 7 and 8, the present invention is dimensioned so as to fit within a compression cylinder of a spring powered gas operated weapon such as an air rifle or an air pistol. In FIGS. 6, 7 and 8 the piston head of the present invention is adapted for a compression cylinder which is circular in cross section.

In this preferred embodiment of the present invention the head (5) and plunger (3) (which jointly comprise a piston end) are substantially as described above and as illustrated in FIGS. 1 to 5 and are both substantially cylindrical with a common central or longitudinal axis.

Passing circumferentially around the plunger (3) adjacent to the intersection of the plunger (3) and the head (5) is an O-ring (22) or other ring or washer. Most preferably the O-ring (22) or other ring or washer is comprised of an elastically deformable material.

The body (2) and extended boss (4) (which jointly comprise the spring abutment means) are constructed as follows. The body (2) is dimensioned so as to have substantially the same external diameter as the head (5) of the piston end and the internal diameter of the compression cylinder (not illustrated) of the spring powered gas operated weapon. The body (2) of the spring abutment means has a depth in the direction parallel to the longitudinal axis of the compression tube that is sufficient for the body to be rigid and to resist the forces to which it will be exposed in operation of the spring powered gas operated weapon, in particular the forces exerted on it by the compression spring (not illustrated) of the spring powered gas operated weapon.

Emerging from the body (2) in a direction parallel to the longitudinal axis of the body (2) is the extended boss (4). The extended boss (4) is cylindrical. The external diameter of the extended boss (4) is both smaller than the internal diameter of the helical compression spring (not illustrated in FIGS. 6 and 7) of the spring powered gas operated weapon and large enough to accommodate a cylindrical blind bore (20) into which the plunger (3) of the piston end may extend. The depth of the cylindrical bore (20) from the surface of the body (2) through which the blind bore (20) passes to the blind end of the bore is greater than the length (in the direction parallel to the longitudinal axis of the body (2)) of the plunger (3).

The O-ring (22) is most preferably so dimensioned that if the plunger (3) of the piston end is substantially fully within the blind bore (20), the O-ring (22) will abut the face of the body (2) that is facing the head (5) of the piston end. The blind bore (20) is in one embodiment most preferably at least partially filled with a viscous fluid and/or a compressible polymeric material, at the blind end of the cylindrical bore (20). In an alternative embodiment, the portion of the blind bore (20) not occupied by the plunger (3) is full of a gas such as air.

As in the previous embodiment the plunger (3) includes a number of circumferential groves or glands (22) in which O-rings (6) may be seated. This allows a tight sliding seal to be made between the plunger (3) and the wall of the cylindrical bore (20). Where the portion of the blind bore (20) not occupied by the plunger (3) is full of a gas such as air the sliding seal is gas tight. The viscous fluid, compressible polymeric material or gas in the cylindrical bore (20) serves to damp motion or absorb kinetic energy between the piston end and the spring engagement means when the piston end rapidly decelerates. This leads to a reduction in piston bounce or recoil.

With reference to FIG. 8, a detail of a groove or gland (22) is shown. The head (5) of the piston end is in the direction “A” as shown in FIG. 8. The groove or gland (22) is defined by first and second side walls (24 and 28 respectively) which are joined by a base (26).

The first side wall (24) is closer to the head (5) of the piston end than the second side wall (28). The first side wall (24) is planar and substantially perpendicular to the outer surface of the plunger (3).

The second side will (28) is planar and is orientated to intersect the surface of the plunger (3) at an angle that is not 90°. The portion of the side wall (28) that is closest to the head (5) is the portion which is joined to the base (26) and the portion furthest from the head is the portion which intersects the surface of the plunger (3). The side wall (28) intersects the plunger at an angle (30) as indicated in FIG. 8). The angle (30) is preferably in the range of about 120° to 160°, about 130° to 150°, or most preferably 140° to 150°.

The base (26) of the groove or gland (22) is in the present example concave. In other embodiments, it can be flat and substantially parallel to the surface of the plunger and the central axis of the plunger, or arcuate.

A particular advantage of the construction of the both present embodiment and that described with reference to FIGS. 1 to 5 is that in addition to the reduction of piston bounce as described above, the spring engagement means may rotate about its longitudinal axis (the axis which substantially coincides with the longitudinal axis of the compression cylinder) without transmitting that rotation to the piston end. This is particularly advantageous because it is a known problem with spring powered gas operated weapons that when the spring is allowed to decompress (because the weapon's trigger has been pulled) in addition to the compression spring extending the spring also tries to rotate about its longitudinal axis. In known spring powered gas operated weapons this exerts a torque on the weapon and can lead to the weapon moving in the users hands. The ability of the spring engagement means to rotate without transmitting that rotation to the piston end has the effect that the accuracy and ease of use of a spring powered gas operated weapon that includes the apparatus of the present invention are improved relative to a spring powered gas operated weapon that does not.

A further advantage of all embodiments of the present invention is that, as mentioned above, it has surprisingly been found that a spring powered gas operated weapon which incorporates the apparatus of the present invention will produce the same performance as a spring powered gas operated weapon which does not incorporate the apparatus of the present invention but with a lighter and/or shorter spring. This reduction in the weight and/or length of the spring (which again reduces weight) has been found to have the effect that for weapons where the barrel is used to cock the weapon, a shorter barrel can be used. The ability to use a shorter barrel has the effect that the pellet being shot by the weapon has a shorter lock time (the time between pulling the trigger and the pellet being discharged from the barrel) than a known longer barrelled weapon. This in turn leads to an increase of accuracy. Indeed, it has been found for the second preferred embodiment of the present invention that the lock time can, with a shorter barrel, be substantially reduced so that what little recoil is still produced by the weapon is experienced by the user of the weapon after the pellet has left the barrel. This leads to a substantial increase in accuracy of the weapon. A further, economic advantage of using a shorter barrel is that it saves material which leads to lower material cost for the construction of the weapon.

A further advantage of the present invention is that the head (5) and body (2) both form separate sliding seals with the longitudinal wall of the compression cylinder which leads to a greater efficiency of compression of the air in the compression cylinder when the trigger is pulled and the compression spring decompresses. 

1. A piston head suitable for a spring powered gas operated weapon characterized in that the piston head is comprised of a piston end and a spring abutment, the piston end is comprised of a head and a plunger, the spring abutment is comprised of a body and an extended boss, at least the body of the spring abutment defines a blind bore which extends through at least the body where the blind bore extends from the surface of the body remote from the extended boss and towards the extended boss, the plunger and blind bore are dimensioned to allow the plunger to fit within the blind bore, and the plunger is provided with at least one feature for mounting a seal around the plunger to allow the creation of a seal between the plunger and at least one wall defining the blind bore.
 2. A piston head according to claim 1 in which the body and extended boss of the spring abutment both have substantially cylindrical curved surfaces and the cylindrical curved surfaces of the body and extended boss have a substantially common axis.
 3. A piston head according to claim 2 in which the surface defining the longitudinal extent of the blind bore is substantially cylindrical and the axis of that cylinder is substantially common with the axis of the body and the blind boss.
 4. A piston head according to claim 1, characterized in that the head and the plunger of the piston end both have substantially cylindrical curved surfaces and the cylindrical curved surfaces of the head and the plunger have a substantially common axis.
 5. (canceled)
 6. (canceled)
 7. A piston head according to claim 1, in which the blind bore in the spring abutment is defined by the body and extended boss.
 8. A piston head according to claim 1, in which the depth of the blind bore defined by the spring abutment is equal to or greater than the length of the plunger of the piston end.
 9. A piston head according to claim 1, in which the feature for mounting a seal around the plunger is at least one groove.
 10. A piston head according to claim 9 in which there is at most four grooves.
 11. A piston head according to claim 9 in which at least one groove is closer to the end of the plunger remote from the head of the piston end than it is to the head of the piston end.
 12. A piston head according to claim 9 in which the piston end further comprises an O-ring located in at least one of the grooves.
 13. A piston head according to claim 9 in which at least one of the grooves has a cross-sectional profile such that the groove has first and second side walls and a base, in which both side walls intersect with the base and extend there from to the surface of the plunger, the first side wall is closer to the head of the piston end than the second side wall, and the first side wall is planar and substantially perpendicular to the longitudinal axis of the plunger.
 14. A piston head according to claim 13 in which the second side wall is not parallel to the first side wall and the second side wall is closest to the first side wall at the second side wall's intersection with the base of the groove.
 15. A piston head according to claim 13 in which the base of the groove or gland is concave.
 16. A piston head according to claim 1 in which the intersection of the blind bore and the surface of the body of the spring abutment is at least one of chamfered, beveled, and rounded.
 17. A piston head according to claim 1 in which the piston end further comprises an energy absorbent material surrounding the plunger adjacent the intersection of the plunger and the head of the piston end.
 18. A piston end according to claim 17 in which the energy absorbent material surrounding the plunger is an O-ring.
 19. A spring powered gas operated weapon comprising a barrel, a cylinder, a helical compression spring, and a trigger mechanism, in which the compression spring is located within the cylinder, characterized in that a piston head according to claim 1 is attached to a first end of the spring.
 20. (canceled)
 21. A piston head suitable for a spring powered gas operated weapon, in which: the piston head is comprised of a piston end and a spring abutment, the piston end is comprised of a head and a plunger, the spring abutment is comprised of a body and an extended boss, at least the body of the spring abutment defines a blind bore which extends through at least the body where the blind bore extends from the surface of the body remote from the extended boss and towards the extended boss, the plunger and blind bore are dimensioned to allow the plunger to fit within the blind bore, and the plunger is provided with at least one feature for mounting a seal around the plunger to allow the creation of a seal between the plunger and at least one wall defining the blind bore; the body and extended boss of the spring abutment both have substantially cylindrical curved surfaces and the cylindrical curved surfaces of the body and extended boss have a substantially common axis; the body, extended boss, piston end and plunger all have a substantially common axis when the plunger is within the blind bore; and the radii of the substantially cylindrical surfaces of the boss of the spring abutment and the head of the piston end are substantially the same.
 22. A spring powered gas operated weapon comprising a barrel, a cylinder, a helical compression spring, and a trigger mechanism, in which the compression spring is located within the cylinder, characterized in that a piston head according to claim 21 is attached to a first end of the spring, the radii of the substantially cylindrical surfaces of the boss and piston end are substantially the same, and the substantially cylindrical surfaces of the boss and piston end are in sliding contact with the cylindrical surface defining the compression cylinder of the spring powered gas operated weapon. 